Hydraulic fracturing is used as a method to potentially increase hydrocarbon production in formations, such as sandstone, limestone, dolomite and shale. A well operator performs the following steps prior to hydraulic fracturing: First, the operator drills a wellbore into the formation and, then, he cases and cements the wellbore. Next, to gain access to the formation, the well operator blasts holes through the casing and cement using high explosives—a process called perforating. Then, to fracture the formation, the operator pumps high-pressure fluid through the perforations—typically gelled water or filtered hydrocarbons laden with chemicals, such as acids, surfactants, and proppants—into the wellbore to fracture the formation under immense hydraulic pressure.
Concerns that hydraulic fracturing may contaminate ground water with hydrocarbons from the formation, and/or chemicals associated with the fracturing processes, have recently brought hydraulic fracturing under public and legislative scrutiny. A recent report in the Proceedings of the National Academy of Sciences, entitled Noble gases identify the mechanisms of fugitive gas contamination in drinking-water wells overlying the Marcellus and Barnett Shales, by Thomas H. Darrah, et al. (vol. 111, pages 14076-81, Sep. 30, 2014, referred to herein as “the Darrah paper”) detailed various modes by which hydrocarbons, from hydraulically fractured wells, could escape into groundwater. That paper concluded that the primary mode of contamination is via structural flaws in wellbore casing and cementing.
Several of the modes discussed in the Darrah paper are shown in FIG. 1, which illustrates a well 100 extending into an area 101 of earth. Between the top surface layer 102 and the target formation (a.k.a., producing formation) 103, area 101 may contain several other strata and formations, such as an aquifer 104 and multiple intervening formations 105 and 106. In a typical region of the Barnett shale play in north-central Texas, the target formation 103 may be about 6500-7500 feet below the surface, the aquifer 104 may typically be about 180-225 feet below the surface (located in the upper Trinity Limestone), and the intervening formations 105 and 106 may be various layers of limestone (e.g., Marble Falls Limestone) or shale.
The well 100 generally includes production tubing 107 extending into a wellbore 108. The wellbore 108 is typically cased with a casing string 109 that is cemented to the inner surface of the wellbore via a cemented annulus 110. Well 100 includes a vertical section 111 and a horizontal section 112. Horizontal section 112 contains fractures 113, as created by hydraulic fracturing.
One possible route by which hydrocarbons produced from the target formation 103 may access aquifer 104 is illustrated by arrows 114 and termed herein as a “deformation route.” Intervening formations may include deformations, such as the deformation 115, which can provide a route by which hydrocarbons, from the target formation 103, can travel to aquifer 104. When the formation is fractured during hydraulic fracturing, the generated fractures 113 may facilitate hydrocarbon transfer from the target formation 103 to deformation 115.
A second possible route is illustrated as arrow 116 and is termed herein an “annulus-conducted route.” As shown in FIG. 1, the intervening formation 106 includes a gas-rich pocket 117 that is penetrated by the well 100. Any imperfections in the cemented annulus 110, i.e., cracks or sections that are not adequately sealed between the wellbore and the casing, can provide a route for hydrocarbons to travel from the gas-rich pocket 117 to the aquifer 104. Also, imperfections in the annulus that extend into the target formation 103 can also provide a route for hydrocarbons to escape from the formation 103 to the aquifer 104.
Arrow 118 represents a third contamination route, in which contamination occurs via compromises in the casing 109. If the casing 109 is compromised with structural defects like cracks or holes, then hydrocarbons and fracturing fluids can escape into the aquifer 104 through those defects. That route is referred to herein as the “casing route.” The Darrah paper concluded that the annulus conducted route 116 and the casing route 118 are primarily responsible for hydrocarbon contamination of ground water associated with the hydraulically fractured wells examined in that paper.
Another problem with hydraulic fracturing is that it requires massive amounts of water—amounts measured in millions of gallons for a single well. Water is in short supply in many areas where hydrocarbon production occurs, and the high water demand associated with hydraulic fracturing imposes a tremendous burden on municipalities in those areas. Moreover, the well operator must install an infrastructure for handling the water to be used for hydraulic fracturing, for storing that water, and mixing it with chemicals, such as acids, gels, foamers, foam breakers, salts, and other adjuvants. The spent fluids, which have been used for hydraulic fracturing, must also be stored, usually in large impoundment ponds, until the fluids can be remediated or disposed of.
The embodiments of the present invention provide in situ formation enhancement apparatus and methods, which are usable for penetrating into a formation and releasing hydrocarbons contained therein, and which solve the problems associated with damage to the wellbore due to the use of explosives and contamination of the surroundings.